1. Field of the Invention
This invention relates to a device for detecting magnetism, which device comprises a thin film of a ferromagnetic magnetoresistive element mounted on a substrate as a magnetism detecting means.
2. Description of the Related Art
A device for detecting magnetism which comprises a thin film of a ferromagnetic magnetoresistive element consisting mainly of a ferromagnetic body and mounted on a substrate as a magnetism detecting means is known.
A device for detecting magnetism is generally constructed such that a variation of magnetism can be detected, for example, as a variation in an electric voltage, utilizing the phenomenon that a value of the resistance of the ferromagnetic magnetoresistive element is changed in the presence of magnetism (an electric field).
The output signal output from such a device for detecting magnetism as described above is usually so weak that these output signals must be amplified by a separate device, for example, an amplifying IC or the like, produced in a separate production process.
Nevertheless, the device is easily affected by the influence of noise, and therefore, a highly sensitive device for detecting magnetism is still required.
As described above, since the output signal from such a device is usually very weak, the output signal from the ferromagnetic magnetoresistive element 101 is amplified by an IC 100, such as an amplifier circuit, a waveform shaping circuit or the like, as shown in FIG. 2.; in which leads 102 and a molded case 103 are also shown.
Recently, a demand has arisen for such an aforementioned IC and ferromagnetic magnetoresistive element to be integrated as one body.
Hereafter, one example of a manufacturing process of a device for detecting magnetism having such an IC and the ferromagnetic magnetoresistive element being integrated as one body, with which the inventors of this invention had tried to make and to use thereof for experiments and tests, will be explained with reference to FIG. 3.
FIG. 3 is a cross sectional view of an example of the construction of a device for detecting magnetism in which the ferromagnetic magnetoresistive element and IC are integrated into one package.
The steps for manufacturing the device for detecting magnetism will be explained hereunder with reference to FIG. 5.
First, in step 200, a P type silicon (Si) wafer 1 is obtained, then in step 201, a thermal oxide film is formed of the P type silicon (Si) wafer by a thermal oxidation process and openings are provided in a predetermined region of the thermal oxide film.
Then, in step 202, an N.sup.+ type buried layer 2 is formed by diffusing antimony (Sb) or arsenic (As) into the wafer 1 from the opening, and in step 203, the 5 thermal oxide film is removed and an N.sup.- type epitaxial layer 3 having a low concentration of impurities is formed on the P type silicon wafer.
Then, in step 204, a thermal oxidation process is applied to the surface of the epitaxial layer 3 to form a thermal oxide film, openings are provided in the film at the position corresponding to the position in the epitaxial layer 3 in which the isolating regions will be formed, and then in step 205, an isolating region 4 is formed by a diffusion of boron (B).
Next, in step 206, an insulating film consisting of SiO.sub.2 is formed by a thermal oxidation process, and then in step 207, a P.sup.+ type diffusion layer 6, which is used as a base region, is formed by diffusing the boron (B) into the N.sup.- type epitaxial layer 3, and successively, in step 208, N.sup.+ type diffusion layers 7, which are used as an emitter region and a contact region with the collector (the epitaxial layer 3), respectively, are formed by diffusing P (phosphorus) into the respective layer in the same way as for forming the P.sup.+ type diffusion layer 6.
Further, in step 209, a contacting portion is provided by selectively forming an opening in the insulating film 5.
The insulating film 5 thus produced is shown in FIG. 3.
Then, in step 210, the contacts are completed by depositing Al thereon to form wiring 9, and in step 211 a thermal-treatment is carried out.
Thereafter, in step 212, the ferromagnetic body including Ni as a main component thereof, and further including Fe and/or Co, for example, an alloy of Ni--Fe or Ni--Co, is deposited on the surface thereof by a vacuum deposition method to form a thin film of the ferromagnetic magnetoresistive element 10 having a thickness of 200 to 2000 .ANG., and then, the thin film is etched to form a predetermined pattern thereon.
Further, in step 213, a surface protecting film 11 which protects the bipolar IC and the ferromagnetic magnetoresistive element thus produced is formed on the surface thereof, and then an opening 2 is formed by removing a portion of the surface protecting film 11 which corresponds to the position at which a terminal of an electrode will be mounted and then, in step 214, the terminals of the electrodes are formed at that portion.
Finally, at step 215, a thermal treatment for recovering the characteristics of the transistor varied due to the formation of the ferromagnetic magnetoresistive element 10 and the surface protecting film 11, and to improve the quality of the film of the ferromagnetic magnetoresistive element 10, is applied to the thusproduced device.
In this manufacturing process, the step 208 for forming the N.sup.+ type diffusion layers 7 of the bipolar IC is carried out by a vapour diffusion method, usually by employing POCl.sub.3 gas, but in the above process, the diffusion source P is also diffused into the interposed insulating film 5 to form a layer 8 in the surface portion of the interposed insulating film 5, in which the concentration of the impurities is higher than that of the rest of the portion of the film 5.
Namely, the layer 8 is not formed as a separate film to the film 5, but is a layer containing a high concentration of impurities therein and is similar to a Phospho Silicate Glass film (PSG film), and accordingly this layer 8 is referred to as a PSG layer hereinafter. On the other hand the interposed insulating film 5 may be referred to as the interposed layer or only simply to the layer hereafter.
This PSG layer 8 has a gettering function, and therefore, in an ordinary IC, usually remains on the surface thereof without change.
But, in the device for detecting magnetism as mentioned above, it was found that, when the ferromagnetic magnetoresistive element 10 is formed on the surface of the PSG layer 8, the quality of the film thus formed, in relation to the electromagnetic characteristics, for example, is remarkably deteriorated when compared with the film of the ferromagnetic magnetoresistive element deposited on the surface of the SiO.sub.2 film formed on the surface of the silicon (Si) wafer only by applying the thermal oxidation process to the surface thereof, without forming a bipolar IC or the like on the surface or on the surface of a glass substrate.
According to experiments by the inventors, it was found that the closer the portion of the ferromagnetic magnetoresistive element 10 is to the insulating film or the layer 5 of the IC, the greater the deterioration of the quality of the film of the ferromagnetic magnetoresistive element.
It was also found that this deterioration is caused by the chemical reaction of P (phosphorus) contained as impurities in the PSG layer 8 formed in the surface portion of the insulating film or layer 5 of the IC with the ferromagnetic magnetoresistive element, due to the thermal treatment carried out in step 215.
It was further found that this phenomenon applies to an insulating film 5 containing only As as the impurities, and not containing P as mentioned above.
According to further experiments concerning the factors causing the generation of noise, depending upon the mechanical construction of the device for detecting magnetism per se, by the inventors of this invention, it was found that, for example, Barkhausen noise is generated when a surface roughness of the surface of an interposed layer of the ferromagnetic magnetoresistive element is coarse.